Enclosures for internal combustion engines, such as those used in mobile power stations, are designed to minimize the sound levels outside the engine compartment. Typically this is accomplished by adding sound insulation to the wall of the enclosure. However, the engine enclosure must allow for sufficient inlet and exit airflow capacity to support combustion and to provide cooling air for the engine. Air inlets are provided by removing sections of the enclosure walls, thereby significantly increasing the sound levels outside the compartment.
It is known in the art to cover the enclosure air inlets and exits with sound dampening absorptive material, typically in the shape of straight or V-shaped louvers. However, engines emit sound based on firing pulses and other periodic occurrences such as cams, gears, and piston slap which generate considerable low frequency sound which, correspondingly have long wavelengths. Absorptive sound attenuators operate primarily at higher frequencies but are not effective at low frequencies. For example, to effectively attenuate 125 Hz sound using absorptive methods, such as a duct silencer or louver, would require approximately a 9 foot deep silencer. Due to limited space inside the engine enclosure, large commercially available duct-type silencer used to draw cooling and combustion air through the enclosure while attenuating noise through the opening are not practical.
A second problem with this type of silencer is that a trade-off must be made between the desired noise attenuation and the pressure drop across the panel. If the louvers are placed in close proximity, substantial broadband (mid- to high-frequency) noise reduction can be accomplished; however, close spacing increases the pressure loss across the panel and thereby reduces the air flow through the engine compartment.
Reactive silencers are common in engine exhaust mufflers and offer good broadband frequency performance. Unfortunately, this technology is highly restrictive to airflow and would also require a large unit for the volumes of air required to flow through it to provide both combustion and cooling air.
For sound attenuation at low frequencies, there are various principles, whose applications have been used and are used in sound attenuators, as is well known. One well known solution for sound attenuation at low frequencies is the Helmholtz resonator. The resonator consists of an air space which communicates with the "outer air" through an opening. An air plug present in the opening forms the mass that resonates on support of the spring force formed by the air enclosed in the hollow space. The resonant frequency of the Helmholtz resonator depends on the area of the opening, on the volume of the air space, and on the length of the air plug formed in the opening. When the volume of the air space becomes larger, the resonance frequency is shifted toward lower frequencies. When the area of the opening is made smaller, the resonance frequency is shifted towards lower frequencies.
When Helmholtz resonators are driven with acoustic energy at the resonant frequency, the resonators will absorb a maximum amount of the incoming acoustic energy. However, because they are tuned systems, the absorption decreases rapidly as the frequency of the incoming acoustic energy varies substantially from the resonant frequency. Thus, the principle limitation with these devices is that they attenuate sound energy efficiently only within a narrow frequency range centered at their tuned frequency.
The present invention is directed to overcome one or more of the problems as set forth above.